The present invention relates to thyristors and more particularly to circuit configurations each including at least one thyristor, which circuit configurations are connectible in series in order to obtain a resultant series circuit in which the desired characteristics are optimized.
Normally the following points are of primary consideration when considering the use in connection of thyristors:
I. Equalization of the various thyristor properties which result from the voltage distribution.
II. The most practical distribution of total voltage on the individual thyristors in accordance with external voltage stress, as well as voltage limitation of the individual thyristors, particularly with respect to maintaining surge voltage in the most practical range of the surge front with minimum loss.
In more specific terms, circuit arrangements employing thyristors should ideally meet the following criteria:
1. Voltage distribution in accordance with operating frequency.
2. Take-up of the storage load.
3. Limitation of the commutation voltage.
4. Voltage limitation at the thyristor.
5. dv/dt-limitation by surge voltage load.
A great number of various circuit arrangements for thyristor circuits have been proposed. Typical of these are the following three circuits (shown in FIGS. 1-3 herein).
FIG. 1 shows a circuit configuration in which R1-C1 and R2 branches are connected in parallel with one another and with a thyristor T. Note, for example, the ASEA Publication 16 (1971) Vol. 2, page 39 entitled "Thyristor Values For The Expanded HGU-Gotland Arrangement". The circuit configuration of FIG. 1 fulfills to a limited extent the above requirements 1 and 2, but for a number of reasons is not voltage limiting.
The arrangement of FIG. 2 includes three parallel paths consisting of the elements R1-C1, R2-C2 and R3, respectively, which are connected in parallel with one another and with thyristor T. The resulting configuration is useful in a larger frequency range compared with the circuit of FIG. 1. However, the circuit of FIG. 2 is not voltage limiting (see CIGRE International Conference on Large High-Tension Electric Systems-1972 Session Aug. 28-Sept. 6, pages 14 and 61).
Finally, FIG. 3 shows the circuit configuration of the HGU-valve for the Power Plant "Cabora Bassa" in Mozambique, which is a cooperative development of the HGUworking agreement of AEG, BBC and Seimans. The circuit of FIG. 3 fulfills requirements 1-3 above. The voltage limitation of thyristor T (requirement 4) comes about through the use of a CAD element condition ("Controlled Avalanche Diode" .tbd. diode which Homogeneous Avalanche Breakdown), while requirement 5, the dv/dt limitation by surge voltage load is obtained through the diode D and the R3-C2 circuit branch.